INSIGHT ON THE ORIGIN OF THE SUR SERIES WITHIN THE SALINIAN TERRANE, CALIFORNIA EVALUATED THROUGH PETROGRAPHY, ZIRCON LA-ICPMS U-PB GEOCHRONOLOGY, AND HF ISOTOPES

Samples collected from Paleozoic-Mesozoic (?) metasedimentary and metaigneous rocks of the western California Salinian Terrane, commonly referred to as the Sur Series, are evaluated utilizing petrography, zircon LA-ICPMS U-Pb geochronology and Hf isotopes. The majority of these rocks are metasedimentary and have a poorly constrained depositional history and few researchers have explored their provenance. Sur Series metasediments are traditionally believed to be part of the southwestern US Cordilleran miogeocline and potentially the associated Mesozoic cover strata. We have also collected and analyzed a select few igneous and metaigneous units to provide further constraints on associated magmatic and metamorphic events. Petrographic analysis on samples collected from nearly 300 km of the California central coast suggests a high degree of variability in the lithologies of Sur Series metasediments, ranging from schists, gneisses, marbles to subordinate quartzites and metaigneous rocks. Additionally, four samples collected from Bodega Head were previously proposed to be Sur Series xenoliths, but unimodal zircon U-Pb ages of 97.8 ± 1 Ma suggest these are enclaves. Detrital zircon U-Pb geochronology of the metasediments suggests a high degree of variability in both the youngest age group of zircon grains and the provenance of the samples. The youngest age groups of zircon grains from the sampled units range from Proterozoic to Cretaceous.

Analysis of Hf isotopes on zircon from six samples provides further information on the origin of the zircon grains. Neoproterozoic and Cretaceous U-Pb ages were targeted for their potential importance to provenance determination and insights into magmatic evolution of potential source regions. εHf (T) from the Neoproterozoic zircons range from 5 to -4.2 and Cretaceous zircons range from 6.5 to -34.4.

The petrography, CL images, zircon LA-ICPMS U-Pb geochronology and Hf isotopes of Sur Series samples provide additional constraints on the geologic history of the Sur Series within the central California Salinian Terrane. Although these data provide some clarity to the compositional variations, depositional ages, and provenance of Sur Series units with the Salinian Terrane, further research is needed to confidently resolve these issues.